Geoscience Reference
In-Depth Information
When the weathering products obtained are soluble, they have to be
eliminated for the reactions to proceed. Thus, well drained environments
are favourable for a strong desilication going up to gibbsite. Gibbsite is
produced at the top slope, for example, in toposequences. Poorly drained
environments at the bottom of toposequences are, on the contrary,
favourable for retention of silica and thus of 2/1 clay minerals, especially
of the smectite type. However, bauxites are often found in basins. On the
one hand they could have been transported there. On the other hand,
under fully water-saturated conditions, the equilibria of the reactions
presented in Table 3.3 (§ 3.3.1) are displaced to the right. This could be
sufficient for bauxitization to occur (Tardy 1993).
Drainage conditions
The weathering reactions are, overall, accelerated by high temperatures.
Also, the solubility of silica increases by 40 per cent from 25º C to 35º C
(Thiry
et al
. 1999). All of this favours the total hydrolysis observed
mainly in the humid intertropical environment.
Temperature
Acidity (therefore pH) is involved in may ways. On the one hand, it
provides the protons necessary for certain reactions and determines the
rate of attack on the primary minerals. On the other hand, it controls
the solubility of the ions obtained, the conditions for their elimination
and, therefore, the progress of the weathering reactions.
The pH is influenced by several factors:
nature of the rock (acid or basic),
progress of weathering, that is the degree of loss of basic
cations,
type of biological activity, which leads to organic compounds,
releasing more protons or fewer,
content of CO
2
in the environment, linked to the respiration of
microorganisms and incorporation of atmospheric CO
2
.
In poorly organic soils, the pH is equilibrated as a function of
atmospheric CO
2
. Actually,
CO
2
+ H
2
O
´
H
2
CO
3
´
HCO
3
-
+
H
+
,
and
pH
=
3.905
-
½
log PCO
2
As the atmospheric
PCO
2
is 3
×
10
-4
or 10
-3.5
, the pH of the rainwater
is theoretically 3.905 + 1.75 = 5.65. Actually, the drop of water that
Conditions of pH
